Three major asepcts of the leucine operon of enteric bacteria will be investigated. (1) The neucleotide sequences of the operons from Salmonella typhimurium and Escherichia coli will be determined. Alterations in nucleotide sequence caused by several interesting mutations will be examined. Together with information from other systems, these studies will provide an insight into the nature of promotors, terminators, intracistronic regions, and the genetic code. (2) The relationship between the levels of leu operon enzymes (and mRNA) and the intracellular concentration of leucine will be determined (operon response pattern). (3) A number of specific mutations will be introduced into the leu leader and the effect of these mutations upon the operon response pattern will be determined. The purposes of thse studies is to determine how the structure of the leu leader region contributes to the regulation of the operon by transcription attenuation.